Dynamics of vibrational excitation in the C₆₀ single-molecule transistor

Abstract

We investigate the vibrational excitations recently observed in the single C₆₀ molecule transistor [Park et al., Nature (London) 47, 57 (2000)]. There can be two mechanisms for this: (a) the displacement of the equilibrium position, as the electron hops onto C₆₀ to form C₆₀^- and (b) the position dependence of the hopping matrix element. We find that if the two electrodes are planar with the C₆₀ sitting symmetrically between the two, then mechanism (a) is not possible, though (b) is, but the results are not in agreement with experiments. Considering C₆₀ to be trapped between a protrusion and a flat electrode, both the mechanisms contribute and the contribution from the second can be large. For example, in the case of C₆₀ trapped between the a protrusion and a flat electrode, the contribution can be as large as 20%. Though our results do qualitatively explain the results, for quantitative agreement with experiments, it seems necessary to consider perhaps the nonuniformity of the charge distribution in C₆₀^- caused by the image interaction or more complex electrode geometries.